The process described in the present patent application does not use a recirculation loop of the product in the emulsification unit. The emulsion thus manufactured has both a fine and monodisperse particle size, and does not show any heating that might lead to detrimental hydrolysis of the ASA. An efficient process that is simple to perform, especially on a papermaking production site, for producing an emulsion that will advantageously be used as a sizing agent in the manufacture of paper sheets is thus provided.
In the field of papers and other cardboards, “sizing” operations are aimed to give these supports improved properties, especially in terms of hydrophobization and resistance to the penetration of hydrophilic species such as water and aqueous inks. In this regard, use is made of “sizing” compositions that contain hydrophobic substances.
Such compositions are equally used as a mixture with the fibrous mass of cellulose that constitutes the structure of cardboard or paper (internal sizing) or as an application to at least one of the faces of this structure (external sizing, sizing, surfacing or coating). The present invention relates here to the exclusive field of internal sizing. For terminological convenience, the simple term “sizing” will denote the term “internal sizing” as defined above.
One of the compounds frequently used in sizing compositions is alkenylsuccinic anhydride or ASA. This chemical species, which is immiscible in water, must be emulsified in order to be used advantageously in the form of a liquid product: good contact between the ASA and the cellulose fibers is thus allowed.
To perform this emulsification, it is known practice to use concomitantly aqueous solutions of cationic starchy materials of different nature, the starchy material being optionally modified; the function of such compositions is to avoid coalescence of the ASA particles by positive ionization of the surface of the particles, and to bring the ASA particles close to the fibers via an ionic mechanism. Broadly speaking, a cationic starchy material/ASA dry weight ratio of between 0.2 and 4 is used.
Such liquid compositions based on ASA and cationic starchy material are especially reported in documents WO 96/35840 A1 and WO 97/35068 A1. They optionally contain surfactants that increase the dispersibility of the ASA, these substances nevertheless being able to interact negatively with the ASA according to the teaching of document WO 97/35068 A1.
Besides the capacity of giving improved properties to the final product, the emulsion of ASA in the aqueous solution of cationic starchy material must have a certain number of characteristics. It must especially have great acuteness of particle sizes, and also a narrow distribution spectrum of these sizes (“monodisperse” product). As explained in document WO 97/35068 A1, these parameters condition the efficacy of the sizing composition with regard to the hydrophobicity properties that it is supposed to impart.
In this respect, it is well known that the presence of “coarse” particles is a source of fouling, especially of the various items of equipment in which the sizing composition transits, but also of the dryer section of the paper machine by steam entrainment of these coarse particles (which may occasionally lead to fires). Conversely, particles of said composition that are too “fine” will pass through the fibrous mattress and will be carried away in the process waters during draining. It is thus necessary to have a sizing composition in the form of an emulsion that has a maximum number of particles whose diameter is centered on an optimum size that a person skilled in the art estimates at between 1 μm and 1.5 μm.
In order to determine the particle size distribution, use is generally made of a laser particle size analyzer which allows counting, by number or by volume, the particles having a certain diameter, or having a mean diameter within a certain range: this is then referred to as particle size distribution and particle population as a function of the range under consideration. In the present patent application, the term “narrow particle size distribution” will be used when at least 80% by volume of said particles have a diameter of less than 2 μm, and when the mean diameter is between 1 μm and 1.5 μm.
The first of these characteristics reflects a reduced proportion of “coarse” particles (diameter greater than 2 μm). By specifying that the mean diameter is within a range which excludes particles that are too fine (whose mean diameter is less than 1 μm), a “narrow” and “monodisperse” particle size distribution is clearly defined, this distribution being centered on the range from 1 μm to 1.5 μm. Moreover, it is specified that in the present patent application, the particle sizes are always measured using a laser particle size analyzer sold by the company Malvern under the name Mastersizer® 2000. The corresponding measuring procedure is reported in the experimental section of the present document.
The prior art mentions a certain number of documents relating to devices and methods for producing emulsions of ASA in an aqueous solution of cationic starchy material. The general principle is as follows: preparing in a first stage an aqueous solution of cationic starchy material, mixing it homogeneously with ASA, and finally preparing an emulsion from this mixture of ASA and of this aqueous solution of cationic starchy material in an emulsification unit. This unit is characterized by the presence of mechanical means of milling or shearing, which micronize and disperse the particles.
In order to prepare such an emulsion having a narrow particle size distribution, the person skilled in the art has for a long time realized that one of the keys of the process was based on the energy employed for the actual emulsification operation, but also in a system for recirculating this emulsion in the emulsification unit. Intuitively, it is understood that this recirculation loop allows many passes of the product into the emulsification unit, which facilitates the micronization process and thus increases the particle dispersion.
As examples illustrating this concept, reference may be made notably to documents U.S. Pat. No. 6,207,719 and U.S. Pat. No. 5,653,915, which directly concern the preparation of an ASA emulsion, using various devices. It clearly appears that the principle of recirculation of the product in the emulsification means is information that has been acquired and integrated by the person skilled in the art (see notably step D of claim 1 of the first document, and step C of claim 1 of the second document). As regards the documents already mentioned, document WO 96/35840 A1 remains silent regarding the devices used, whereas document WO 97/35068 A1 mentions a Gaulin mixer in its examples, this mixer being known to have a recirculation loop.